宽度40px，另一个的补全宽度：
<!DOCTYPE html> <html lang="en"> <head> <meta charset="UTF-8"> <title>Document</title> <style> html,body,.box{ height:100%; } body{ margin:0; } .box{ display:-webkit-flex; -webkit-flex-direction: row; display:flex; flex-direction:row; } .box .child{ width:40px; background:#f00; } .box .child1{ flex:1; background:#0f0 } </style> </head> <body> <div class="box"> <div class="child">child</div> <div class="child1">child1</div> </div> </body> </html>